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I agree with you with about everything besides the lack of coverage, Primarina with a Water STAB/Moonblast/HP fire/Energy Ball or Ice Beam gives a really good coverage. Psyquic is another good option.
The only thing that could made primarina coverage better in my opinion is focus blast.
Primarina don't have the tools to be a efficient support pokemon, so the only way to run it is as bulky attacker.
With the pixelate nerf (the bonus of pixelate was nerf to 20%), on a choice specs set, sylveon's hyper voice doesn't hit harder that primarina's moonblast anymore.
-252+ SpA Choice...
On the description of psyquic terrain it says that it prevents priority moves, which makes me believe it will not only cancel attacks but every priority move.
That may turn out to be another to counter spot light, but also means rip protect.
Hmm... I don't think liquid voice will be that bad for doubles, it doesn't get the 30% boost but still gives stab to hyper voice, making it a safer way to hit all the pokemons on the opponent side than surf. The other option is dazzling gleam.
Also sorry for the avatar.
Sorry for asking, I just would like that someone make, explain to me, shortly and plain, why delphox have no niche on the OU tier, I am not saying that the pokemon is OU, i just want understand why.